According to the conventional microwave oven, it is designed to control the output of the magnetron by switching the alternating current voltage inputted to the high voltage transformer through the triac or relay so that it is possible to minimize the excess inrush current by carrying out a switching operation at the .pi./2 crossing point of the alternating current input voltage.
In a case where the triac is used for controlling the power supply, it is possible to carry out an exact switching at the .pi./2 crossing point of the alternating current input voltage due to a rapid and constant answering speed of the triac.
However, in such a case, there is a disadvantage in that it needs a radiating plate for radiating the heat produced from the triac, which makes the whole structure more complicated, and moreover, since the triac is expensive, the cost is higher.
Furthermore, in a case where a relay is used for controlling the power supply, there is also a disadvantage in that the answering speed of the relay is late and the answering speed is not constant depending on the characteristics of the relay. Therefore, it is impossible to carry out an exact switching at the .pi./2 crossing point of the alternating current input voltage and an excess inrush current is thereby increased at the time of switching. At this moment, a driving signal of the relay is output for a constant period, the drive of relay is continuously made for only one of the positive or negative cycles of the alternating current input voltage so that a phenomenon of potential difference takes place only in a direction between contacts of the relay, and thereby the life of the contacts of the relay is shortened.